The present invention relates to automobile wheel alignment systems. In particular, the invention relates to a method and apparatus for mounting wheel alignment test equipment on a wheel of an automobile.
Proper alignment of wheels in an automotive vehicle is important for proper handling of the vehicle and proper tire wear. In addition, the proper alignment of the wheels will decrease fuel consumption as well as increase vehicle safety. The wheel alignment parameters which are measured and adjusted in order to achieve proper wheel alignment are camber, caster, steering axis inclination and toe.
Accurate measurement of these parameters is only possible if the test equipment is accurately and securely coupled to the wheels of the vehicle under test. Minor inaccuracies in the connection of the test equipment to the automobile wheels can give rise to errors in the alignment measurements. For accurate alignment, the axial registration between test equipment and the wheels must be maintained.
In measuring wheel alignment, measuring devices ("head units") are clamped onto the four wheels of the vehicle under test. On each side of the vehicle, head units measure the front to back and back to front angles between each pair of wheels. The head units also measure the side angles between the front or back pair of left and right wheels. U.S. Pat. No. 4,594,789 issued June 17, 1986 to Marino et al entitled "Wheel Alignment System" is incorporated by reference. The Marino patent shows a wheel alignment system using head units to measure wheel alignment angles. The head units are mounted to the wheels with a clamping apparatus called a "rim clamp." Typically, a rim clamp clamps to a rim of a wheel. The rim clamp provides a spindle generally co-axial with an axis of the wheel. The spindle is inserted into a spindle clamp on the head unit. The spindle clamp includes a set screw which can be tightened against the spindle to secure the head unit on the rim clamp.
During alignment measurements for the vehicle under test, the wheel must be rotated. For example, when an operator performs run-out compensation. To rotate a wheel, the set screw which secures the head unit to the rim clamp must be loosened. The wheel is then rotated while the head unit remains generally stationary relative to the other head units and the body of the car. However, during this procedure, the head unit can slide along the axis of the spindle so that the registration in the axial direction between the head and unit rim clamp is lost. Loss of registration introduces an additional angle into the alignment measurements which causes an error.
It would be highly desirable to provide a mount for a wheel unit in a automotive vehicle wheel alignment system which allows the wheel to be rotated without loss for registration in the axial direction between the head unit and the wheel.